1. Field of the Invention
The present invention relates to a dry presensitized plate, more particularly to a dry presensitized plate for use in making a dry lithographic printing plate requiring no dampening water and having an extremely wide developing latitude as well as a method of manufacturing the same.
2. Description of the Prior Art
The conventional lithographic printing technique is characterized in that water applied to non-image areas of a printing surface is not compatible with oil printing ink or it serves as an ink repellent.
However, in the conventional lithographic printing technique, there have been a number of problems accompanied by the fact that the dampening water and the oil ink coexist with each other on a printing matrix. For example, a part of the dampening water to be applied to the matrix causes back-flow in a printing machine during printing procedures, which, in turn, causes the contamination and emulsification of the oil ink. Moreover, the dampening water flows through the offset drum and gets printing papers wet which results in the curling and dimension change of the paper. Furthermore, it is, in general, required to maintain a delicate equilibrium between the amount of dampening water and oil ink so as to obtain a uniform and precise image, the equilibrium being one of the most important factors, in the wet lithographic printing technique and as a result a desired quality of printed product are assured.
Under these circumstances, a dry presensitized plate has recently been proposed and widely put into practical use. Such dry presensitized plate generally comprises a substrate and a photosensitive layer and a silicone rubber layer which are deposited, in order, on the surface of the substrate. Examples of the dry presensitized plate having such construction are disclosed in, for instance, Japanese Patent Publication Nos. 23042/1969, 16044/1971, 17081/1976, 26923/1979 and 22781/1980.
Among them, the dry presensitized plate disclosed in Japanese Patent Publication No. 22781/1980, in which a photodimerizable photosensitive resin is used, has an advantageous property such that it is not influenced with oxygen during the plate making process and the exposure process and that it is not influenced by the exposure temperature, thereby such plate makes it possible to manufacture the lithographic printing plate more stably than a plate in which a photopolymerizable composition is utilized.
There have been proposed two methods for making such a dry presensitized plate, one of which comprises dissolving a photosensitive layer of image areas with a developing liquid (developer) to remove a silicone rubber layer deposited thereon and thus obtaining the image areas (see, Japanese Patent Publication No. 16044/1971) and the other of which comprises selectively removing a silicone rubber layer of image areas utilizing the photoadherent property and photorelease property of a photosensitive layer (see, Japanese Patent Publication No. 26923/1979 and Japanese Patent Laid-Open Appln. No. 80046/1981).
The former presents an advantage such that the development is not affected by the bonding strength between the silicone rubber layer and the photosensitive layer. This is because, the removal of the photosensitive layer is carried out by dissolving the same with a developer, in this method. Thus, according to this method, it is expected to produce a printing plate having a rather high resistance to scratch and a high wear resistance during development and/or press operations without impairing the developing property thereof.
However, in the latter method, the printing plate produced has not an excellent developing property, since the developing property is strongly influenced by the bonding strength between the silicone rubber layer and the photosensitive layer. Therefore, it is difficult to obtain a printing plate excellent in its resistant to scratches and wear resistance according to this method.
In general, a photosensitive layer should be as thin as possible in such a method wherein the photosensitive layer is removed by dissolving it with, for instance, a developer. In such method, image areas are constituted by concave area formed after the dissolution of both silicone rubber layer and photosensitive layer. Therefore, the concave area of the printing plate becomes quite deep and this causes problems explained below. In such cases, when an amount of ink supplied to the concave areas during printing operation is small, the ink cannot be transferred to printing paper, especially in highlight areas.
Under these circumstances, the inventors of this invention have conducted various studies to overcome the foregoing disadvantages accompanied by the prior art and found that these disadvantages can effectively be eliminated by providing a dry presensitized plate comprising a primer layer composed of a specific material strictly selected, a photodimerizable photosensitive layer and a silicone rubber layer deposited on a substrate in this order. The dry presensitized plate having such structure makes it possible to reduce the thicknesss of the photosensitive layer and to apply ink to the surface thereof in thick. Furthermore, the plate is excellent in its resistance to scratches and wear resistance.
It is sometimes necessary to strongly develop the dry presensitized plate. In such case, the photosensitive layer is peeled off from the primer layer and a good image is not produced in a sufficient reproducibility, that is only a narrow developing latitude is resulted. Alternatively, the inventors of this invention tried to use a silane coupling agent or a mixture thereof in order to enhance the adhesiveness between the primer layer and the photosensitive layer and enlarge the developing latitude, the silane coupler being generally used as the agent for improving the adhesiveness between organic materials (layers); or an organic material and an inorganic material. However, desired effects have not been obtained at all.